BAGHDAD, Aug 30 (Reuters) - Iraqi army and Kurdish forces closed in on Islamic State fighters on Saturday in a push to break the Sunni militants' siege of the Shi'ite town of Amerli, army sources said.

Two officers said Iraqi troops, militia and Kurdish peshmerga were advancing from four directions on the northern town, which has been surrounded by Islamic State forces for more than two months.

Separately on Saturday, a suicide bomber driving a car packed with explosives killed at least 11 people in a town just south of Baghdad.

Armed residents of Amerli have managed to fend off attacks by the Islamic State fighters, who regard its majority Shi'ite Turkman population as apostates. More than 15,000 people remain trapped inside.

A major in the Iraqi army, who was advancing north towards Amerli from Udhaim, said progress was slow because the militants had mined the roads. He said they were around 15 km (9 miles) from the town, while those approaching from the north were just 3 km away.

The major said he had counted the corpses of more than 40 militants killed in Iraqi air strikes on the road between Udhaim and the village of Injana.

Also on Saturday, the Pentagon said U.S. warplanes and armed drones carried out five airstrikes on Islamic State fighters near Iraq's largest dam, the latest in a series of attacks in support of Iraqi and Kurdish forces.

The strikes destroyed an Islamic State armed vehicle, a fighting position and weapons and damaged a building near Mosul Dam, the Pentagon said. Backed by U.S. air power, Kurdish forces recaptured the strategic facility nearly two weeks ago.

Islamic State militants overran most of Sunni Arab Iraq after seizing the northern city of Mosul on June 10, and have proclaimed a caliphate straddling the border with Syria, where they also control vast swathes of territory.

The lightning offensive brought the militants within range of the capital of Iraq's autonomous Kurdistan region earlier this month, prompting air strikes by the United States.

The Kurds have since been slowly regaining ground from the militants and on Saturday advanced on the northern town of Zumar.

Peshmerga spokesman Halgurd Hikmat said control over Zumar would help the Kurds to retake Rabia and Sinjar, two other areas seized by Islamic State.

Violence in Iraq this year has reached levels not seen since 2006-2007 when the country was in the throes of civil war.

The suicide bombing took place at a checkpoint at a northern entrance to the town of Yusifiya, 15 km from the capital, said a police officer, who spoke on condition of anonymity.

"The suicide bomber drove into the checkpoint and blew up his car amongst vehicles waiting to be searched," the officer said. (Additional reporting by Matt Spetalnick in Washington and Isabel Coles in Baghdad; Writing by Isabel Coles; Editing by Andrew Roche, Toni Reinhold)

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